Magnetoresistive sensors have been widely used in applications such as magnetic hard disks and angular transducers in cars. In 1998, Baselt et al. described using functionalized magnetic particles as tags for detecting biological molecules hybridized to probe biomolecules attached to the surface of giant magneto-resistive (GMR) sensors. More details are described in U.S. Pat. No. 5,981,297, which is hereby incorporated by reference in its entirety. Biochips using magnetoresistive devices as sensors are based on detecting biologically functioned magnetic particles of micrometer or nanometer in diameter. The surface of the biochip is functionalized with antibodies or antigens (probe molecules) that recognize and conjugate specifically with the target molecules in the sample applied onto the surface of the biochip. Paramagnetic particles functionalized with the same type of antibodies or antigens as those on the surface of the biochips are then dropped onto the chip surface. Some of the paramagnetic particles conjugate with the target molecules which have hybridized with the probe molecules on the chip surface, forming a sandwich structure. After free magnetic particles are removed, a magnetic field is applied to the biochip to detect to existence of the paramagnetic particles. Target molecules are quantified by the amount of paramagnetic particles detected.
In Addition to Magnetoresistive (GMR) devices, Magnetic Tunnel Junction (MTJ) devices can also be used as the sensing elements for detecting magnetic particles. Such a biochip is described by Xifu Jiang et al. (Korea) in SIPO China patent application Biosensors and Arrays of Sensor Units (Appl. No. 200310113330), which is hereby incorporated by reference in its entirety. Chao Chen et al. proposed the application of MTJ as the sensing element of biochips in SIPO Pat. No. ZL 0213936. X, which is hereby incorporated by reference in its entirety. In SIPO Pat. Appl. No. 200510005035.5, Lei Wang et al. suggested that Si3N4 passivation layer could be replaced by plastic passivation layer.
Researches in the past decade has shown that, as a new technology platform in the development of various bio-sensor chips, magnetoresistive biosensor is a very good candidate for high speed, high sensitivity and high throughput detection of biomolecules at a reasonable cost. At the 2006 Biosensor Congress in Toronto, researchers with Philips reported that there had been more than 30 groups worldwide developing magnetoresistive biosensors.
In order to improve test throughput and the functionality of magnetoresistive biosensor chips, one can enlarge the array size of the magnetoresistive sensors, spot many different kinds of probing molecules on the array, and detect numerous target molecules simultaneously. A practical issue related to such a design is that the number of conductor lines connecting the sensing element and the area occupied by the conductor lines increase proportionally with the array size. The area taken by conductor lines and the complexity related to laying out these lines make large scale integration of magnetoresistive sensor array practically impossible. Additionally, the large number of bonding wires necessary for connecting the conductor lines on the chip to a testing apparatus cause tremendous amount of difficulty for packaging and testing. To overcome these issues, one can integrate the magnetoresistive sensor array with a large scale integrated CMOS circuit. The three dimensional architecture of magnetoresistive sensor array on top of the CMOS circuit makes a biochip with a large array of magnetoresistive sensors feasible. Consequently, detecting a large number of target molecules using a biochip with a large array of magnetoresistive sensors becomes a reality.
Even though there are numerous publications on biochips using magnetoresistive sensors, what is still needed is a testing apparatus for biosensors and biochips using magnetoresistive or magnetic tunnel junction devices as the sensing element, particularly a testing apparatus for integrated CMOS magnetoresistive biochips.